V8 by itself is single threaded (GC, compiler are all performed in the same thread that executes JavaScript) but V8 can be used by two threads (not simultaneously though, they have to maintain exclusive access to V8 isolate).

Nitro Assembler and Mozilla have a scary change coming around called JIT hardening, change code cache permission back to X after code patching and do WX before patching... any idea if V8 intends to do somethign similar

part of the page meddling part has to do with internal processor trace caches (which are really Trace JITs of predecoded instructions) when you change page protections they have to be blindly discarded

we flipped the switch internally fixed some bugs, the perf was atrocious, I guess security triumphs perf. If this is going to be a mainstream browser feature then time for processors to go back and re-add stuff ;-)